1. Field of the Invention
This invention relates to testing and measuring apparatus and more particularly to an apparatus and method for measuring an alignment characteristic such as camber of a vehicle wheel.
2. Description of the Prior Art
Numerous prior patents have addressed the problem of accurately and efficaciously measuring vehicle wheel alignment characteristics such as camber. For example, U.S. Pat. No. 3,199,208 to Hunter discloses a vehicle wheel alignment apparatus for measuring a number of alignment characteristics including camber, caster, king pin inclination, and wheel toe, all of which are defined in that patent.
Camber angles in particular have in the past been determined by devices based upon the use of light beams, such as those shown in U.S. Pat. Nos. 3,337,961 to Holub and 3,552,024 to Hunter. Such light-beam based systems for measuring camber can be of some complexity as shown in U.S. Pat. No. 4,192,074 to Chang. Light-beam based systems are also known for compensating for the various wheel alignment characteristics. For example, an electronic light-beam based scheme for combined toe and camber compensation is disclosed in U.S. Pat. No. 4,274,738 to Hollandsworth et al.
One traditional way of measuring the camber angle of a vehicle wheel has involved the use of a meter movement having a movable arm subjected externally to the force of gravity and internally to a force created by a current flowing through a coil. When the meter movement is oriented vertically, it is affected by gravity. The resulting gravitational force relates to the camber angle since camber is measured with respect to a vertical reference. The type of meter movement or camber transducer most frequently employed to date incorporates two optical interrupters which are used to detect the position as well as direction of movement of the meter arm. In such devices, the optical interrupters are oriented such that the light beam is normal to the arc traversed by the meter movement arm. This arm in the art is also called a vane or paddle. A good example of a system of this sort is found in U.S. Pat. No. 3,892,042 to Senften. When such a device is oriented properly, the lines of sight of the optical interrupters are angularly displaced by the magnitude of the camber angle.
Systems having two optical interrupters inherently suffer from a number of disadvantage however. In such systems, the spacing between the two interrupters and the tolerance of the vane thickness are critical. Meeting such tolerances is a time-consuming and expensive manufacturing problem.
The methods of exercising the meter movement and monitoring the effect of gravity upon the movement arm have evolved over the years. Some early camber transducers applied a proper DC (direct current) current to the coil of the transducer to displace the arm or vane to a position between two spaced light beams of an optical interrupter. The transducer outputs, in turn, controlled the DC current source in a closed loop which formed a classic analog DC servo control system. The camber angle, derived from any point along the closed loop in these systems, was directly proportional to the DC voltage necessary to generate the current for meter movement. Since the meter movement stimulus was from a DC or slowly changing source, movement of the transducer arm was designedly slow. Unfortunately, with such slow movement, it was difficult to adequately and accurately overcome friction and inertia of the meter movement, both of which may differ at different angles due to imperfections in the movement's pivot point. This property could hide small changes in camber when these previous devices were used.
When such a transducer was used with a digital measuring system, the output of the transducer was then converted to digital form by an analog-to-digital converter. The resulting resolution in these systems is limited by the number of bits "n" in the analog-to-digital converter, and the maximum range, "2R", of the transducer, provided that the output of the transducer varies over the entire input operating range of the analog-to-digital converter. The resolution of the measuring system in such a case is 2R/2.sup.n. For a transducer having a maximum range of -10 to +10 degrees, with a ten-bit analog-to-digital converter, the resolution is 20/1024, which equals 0.020 degrees. Using an eight bit analog-to-digital converter in such a system would yield a resolution of 0.078 degrees.
Later systems, in order to eliminate the frictional and inertial problems in the meter movements and to improve the sensitivity of the circuit to small changes in camber, exercised the meter movement with an AC (alternating current) drive. The meter movement arm or vane in these systems dithered back and forth between optical interrupter lines of sight, changing directions based on position detection derived from the optical interrupter outputs. These were classic AC servo control systems. The dither, or vibration, frequency of these systems depended upon the hysteresis in the position detection circuitry, the magnitude of the driving current, and the meter movement characteristics in such systems.
Some older measurement systems included a low pass filter which would generate an average voltage proportional to the average drive current required to maintain equilibrium. This output was then displayed as representative of the camber. This was analogous to the DC servo systems described above.
All the above systems measured and displayed the most recent voltages generated by the camber transducer. The only averaging capabilities of those systems was in any R-C time constants used to filter voltages.